1. Field of the Invention
The invention relates in general to a memory structure, and more particularly to a memory structure which uses reference group bit values to indicate sets of reference currents while sensing memory units.
2. Description of the Related Art
The memory capacity for storing data grows larger and larger recently. With the amount of the memory cells grows higher, the threshold voltage distribution range of the memory cells therefore becomes very large. FIG. 1A and FIG. 1B are examples of threshold voltage distribution diagrams of a conventional 1-Megabites memory and a conventional 1-Gigabites memory respectively. Both of them are 2-level cell memories. The horizontal axis represents the threshold voltage of a memory cell; the vertical axis represents the amount of memory cells. The threshold voltage distribution of the 1-Megabiyte memory includes distribution regions 101 to 104. SW1 is the sensing window between the high boundary of distribution region 101 and low boundary of distribution region 102. Similarly, SW2 is the sensing window between distribution regions 102 and 103. SW3 is the sensing window between distribution regions 103 and 104. Distribution regions 105 to 108 are threshold voltage distribution regions of the 1-Gigabiyte memory. Sensing windows SW4 to SW6 are the sensing windows of the 1-Gigabyte memory. As shown in FIG. 1A and FIG. 1B, the ranges of distribution regions 105 to 108 are generally larger than the ranges of distribution regions 101 to 104, which causes sensing windows SW4 to SW6 of the 1-Gigabyte memory are much narrower than the sensing windows SW1 to SW3 of the 1-Megabyte memory. Thus, when the capacity of a memory grows higher, the diversity of the threshold voltages of the memory cells of the memory becomes larger, and the sensing windows of the memory become narrower, which causes difficulty to perform the sensing process for distinguishing states of memory cells of the memory when reading the memory.